Throughout the history of polymerization reactions, the lack of a simple and reliable method for determining the degree of conversion at any time during the reaction has been a serious obstacle in both production control and research study of the polymerizations. A number of techniques, other than total solids conversion, have been proposed for determining degree of monomer conversion but with limited success. For instance, in certain gaseous monomer systems, it is possible to follow reaction pressure to determine degree of monomer conversion, however, this approach is useful in the final stages of polymerization since pressure remains constant until the separate monomer phase is consumed at the end. Dilatometric measurements of the volume changes during polymerization offer a possibility of a highly precise conversion determination but it would be difficult to apply in laboratory and production reactors at similar pressures and agitation conditions. Approximate reaction rate data can also be obtained by measuring cooling water temperatures and flow rates and thus estimate the rate of heat evolution. Refinements of equipment and instrumentation are, however, needed to enhance the accuracy of this method which further would require accurate integration to provide conversion data.
Certain peculiarities relating to polymerization reactions accentuate the need for a better method for determining degree of monomer conversions than is presently available. For instance, low conversion polyvinyl chloride resins, that are blown down at or before the point of pressure drop in the reaction, have become an important factor. In the face of variable monomer purity and other causes of variable reaction rates, pressure and time of polymerization are not a reliable criterion for obtaining consistent final conversions. Certain polymer products require additions of materials or changes in operating conditions at particular conversions, and in research work, studies of polymerization mechanisms are completely dependent on accurate conversion measurements throughout the course of the polymerization.
The most common method for measuring monomer conversion in large-scale production is the solids method. To determine percent monomer conversion, a sample is extracted from the reactor with a flexible vial of known volume. This is accomplished by attaching the vial to a line, evacuating the vial by squeezing it, drawing the liquid sample by releasing the vial so that it expands and fills with the liquid, and closing the valve on the vial. Next, the vial with the liquid sample is weighed, the sample is deposited in a pre-weighed aluminum pan containing hydroquinone short stop solution, the vial is re-weighed to obtain sample weight by difference and the pan with the sample is placed in a drying oven to evaporate all volatiles, i.e., monomers, water, etc. The pan is re-weighed to determine the weight of solids on the basis of which, percent conversion is calculated. In actual plant practice containing dozens of reactors, it is apparent that determination of the sclids conversion is not a simple matter. During the course of polymerization that can take up to one full day, only a few samples are taken. This can lead to many quality variations especially if it is desired to make an addition of a material or make an operating change at a certain solids conversion, since an accurate solids conversion is not available and cannot be pinpointed. Likewise, delay in analysis, which is inherently present in a plant operation due to the time that is required to take and deliver samples to a laboratory, allows the reaction in the vial to continue unabated with the result that inaccurate data is obtained.
The use of gas chromatography to gauge monomer conversion is also known. This technique, introduced by H. Kahn in 1958 for the study of Ziegler - type diene polymerizations, involves periodic chromatographic analysis of vapor samples to determine the decrease in monomer concentration relative to that of a volatile, inert solvent. The invention disclosed herein pertains to improvements in such a method.